Samurai X(rurouni kenshin) is a good example. but don't even bother with the second season... german black knights and feng shui ninjas.... are big parts of the storyline... and i'm not even joking. just watch the first season of epic, and then watch the ovas. anime producers diverged from the storyline and started taking lsd and pcp for the second season.

Trigun is also a pretty good example. unfortunately both are way less impressive than they could be due to the no kill restrictions, but they can still take out large groups of highly trained people with surprising ease..

EDIT: and i am completely seconding sapphiresky's mention of ansatsu kyoushitsu, manga was hysterical and the guy is the most epic thing to exist in the history of everything XD. i had absolutely no idea it was anime and am thus completely happy you made this thread. TO GOOGLE!!!

...was happy to watch episode 1, was sad to find out that is all there is atm T_T

i dont think the last 2 have any anime adaptions(which was rather disappointing since i was surprised and curious about aiki's lol). anyway pretty sure the "one-man-army anime MC" would usually point to only mentioning one's with an anime adaption